The present invention relates in general to cellular communication systems, and is particularly directed to a time division multiple access (TDMA) cellular system, in which co-channel interference from users in cells adjacent to a cell containing a desired user is effectively canceled, by adaptively modifying the directivity pattern of that cell""s base station phased array antenna, in a manner that dynamically places nulls on those co-channel users whose transmission time slots overlap the time slot of the desired user.
In a time division multiple access (TDMA) cellular communication system, a simplified illustration of which is diagrammatically shown in FIG. 1, communications between a base station BS of a respective cell, such as centroid cell 11, and a user within that cell are subject to potential interference by co-channel transmissions from users in cells dispersed relative to cell 11, particularly immediately adjacent cells, shown at 21-71 as being geographically dispersed around cell 11. This potential for co-channel interference is due to the fact that, in order to make maximal use of system resources, the same transmission frequency is employed by multiple system users, who are assigned respectively different time slots for communications (with the base station of their cell).
In the non-limiting simplified example of FIG. 1, the system employs a frequency reuse of one, wherein a given channel in a cell is subdivided into three time slots, each of which is assigned to a respectively different user among a set of three users per cell, as shown in the time slot diagram of FIG. 2. In this example, co-channel users for the respective cells 21, 31, . . . , 71, surrounding cell 11 are shown at 21-1, 21-2 and 21-3; 31-1, 31-2 and 31-3; 41-1, 41-2 and 41-3; 51-1, 51-2 and 51-3; 61-1, 61-2 and 61-3; and 71-1, 71-2 and 71-3. Cell 11 is shown as having users 11-1, 11-2 and 11-3. In this example, preventing interference with communications between user 11-1 and its base station BS from each co-channel user in the surrounding cells 21-71 appears to be an ominous taskxe2x80x94ostensibly requiring the placement of twelve nulls in the antenna pattern generated by the antenna employed by the centroid cell""s base station BS.
In accordance with the present invention, this seemingly daunting problem is successfully addressed by taking advantage of the fact that, in a TDMA communication system, only a portion of the plurality of co-channel users will be transmitting at any one time (i.e. only during their assigned time slots). As can be seen in the time slot diagram of FIG. 2, for the illustrated example, this means that, during any user""s time slot (such as the time slot TS11-1 associated with user 11-1 in cell 11), no more than six potential co-channel interferers in the immediately adjacent cells 21-71 will be transmitting. Consequently, at any given time, a phased-array antenna configuration containing only seven antenna elements would be sufficient to place a null on each active interferer.
The problem is the fact that, even though, at any instant in time, the number of interferers is no more than a fraction of the total number of co-channel users in all of the adjacent cells (only six of the eighteen in the present example), the participants in that number is dynamically changing, as indicated by the eighteen dotted time slot transition lines passing through each half frame of time slots in FIG. 2. Moreover, the time slot assignments for each cell are not synchronized with those of any other cell.
However, since each cell""s time slot assignments repeat in a cyclical manner, it is possible to periodically update a set of amplitude and phase weights, through which the directivity pattern of a multi-element (seven element) phased array antenna is controlled in a time division multiplexed manner, so as to effectively follow the changing participants in the pool of interferers, and thereby maintain the desired user effectively free of interference from potential interferers in any of the adjacent cells.
To this end, the present invention employs a phased array antenna having a plurality of antenna elements distributed in a two-dimensional (unbalanced) array and capable of controllably generating a directivity pattern, the gain of which may be defined (reduced) in a plurality of directions (one less than the number of elements in the array), and thereby selectively place nulls on each co-channel user of an adjacent cell. In order to properly adjust the antenna weights, knowledge of which co-channel users may be transmitting at any instant in time is required. This information is derived by monitoring transmissions from co-channel users in adjacent cells and processing contents of these monitored transmissions to determine relative offsets between their assigned time slots and the time slot of a desired user.
In a preferred embodiment of the invention, elements of the base station""s phased array are driven by a weighting circuit, to which a set of amplitude and phase weighting coefficients are supplied by a time slot processing unit. In a timing acquisition mode, the antenna array is coupled to the time slot processing unit on the traffic channel, while the channel is idle. This idle time occurs just prior to the desired user being directed to that channel. When the desired user begins transmitting, this timing relationship extracted during this timing acquisition mode is employed to control the directivity pattern of the phased array for data communications for that desired user. Alternatively, the desired user can be nulled until timing is acquired.
The time slot processing unit includes a narrowband digital tuner, which receives an IF output of a downconverter and provides decimated in-phase (I) and quadrature (Q) baseband channels in digital format to a digital signal processor. This processor executes a finite impulse response (FIR) function that is matched to the pulse shape of the received signal and correlates with the embedded synchronization pattern. The finite impulse response function may be implemented as a parallel bank of correlation filters, each of which correlates the subsampled baseband signal with a copy of a respectively different (pseudo noise or PN) synchronization pattern associated with the co-channel interferers of the interfering cells. The correlation filters may be implemented as tapped delay lines to which respective copies of synchronization patterns associated with the co-channel users of the system are coupled. The successive tap weights are the values of a respective synchronization pattern sampled at a rate N/T, where N is the number of samples per symbol and T is the symbol period. Each correlation filter is coupled to an absolute value operator to derive an output representative of the magnitude of the correlation.
The magnitude of this filtered signal is then coupled to a commutated timing selector, which derives the timing of peaks in averaged sets of the correlations of the embedded synchronization patterns associated with the potential co-channel interferers. The timings of these correlation peaks identify times of transitions between successive co-channel user time slots relative to the time of transition of the desired user""s time slot. The timing selector includes a demultiplexer, which functions as a cyclical commutator at a rate of N/T. Running totals of the respective outputs of the demultiplexer are accumulated and stored. The contents of each accumulator memory stage are periodically sampled at every CMT seconds, where C is the number of half frames, M is the number of symbols per half frame.
The averaged data values are then coupled to respective threshold comparators. If a respective data value exceeds a threshold, the output of the comparator is a xe2x80x981xe2x80x99; if a respective data value fails to exceed the threshold, the comparator output is a xe2x80x980xe2x80x99. These threshold outputs are then stored in a timing acquisition memory. The index of the memory stage whose contents indicate a threshold exceedence (e.g. a xe2x80x981xe2x80x99) identifies the time of occurrence of the synchronization pattern of an interferer.
This extracted timing data is coupled to a weighting coefficient control operator, which adjusts a set of values of the amplitude and phase weighting coefficients for each weighting circuit of the antenna array, as necessary, to form a respective null in the phase array""s directivity pattern in the direction of each co-channel interferer whose synchronization pattern correlation peak has been identified by the commutated timing selector.
The antenna""s gain and phase weights are only updated as the participants of a pool of interferers changes, namely, the time slot of one of the co-channel interferers terminates and the time slot of another interferer begins. In addition to being applied to the weighting coefficient circuits, the updated weighting values are stored in memory until the next occurrence of the time slot of the last entry in the current pool of co-channel participants. In response to this next occurrence, the set of weight control values for the current pool is updated and used to adjust the phased array""s directivity pattern. The newly updated weight set is then stored until the next update interval for the current co-channel user pool.
The directivity pattern produced by updated values of the antenna""s weight control elements is maintained by the weighting coefficient control operator, until the next transition between successive time slots of any of the co-channel users in the adjacent cells (corresponding to a new pool of co-channel users, in which the time slot of a previous user has terminated and that of a new user begun in its place).